The present invention relates to a multi-stroke linear actuator capable of achieving a predetermined number of discrete positions, more particularly, it relates to a linear actuator for accurately moving a tooling member a preselected distance.
Many conventional devices are known for guiding and positioning a tool or an element, such as a parts gripper, with respect to a work piece. These devices range from simple hand-operated mechanical devices to more accurate and automatic, fluid operated devices in which the tool can be located in numerous positions by controlling the pressure and amount of the fluid. Such devices are commonly used in a variety of environments to perform a multitude of work functions such as the pick-up placement of parts in assembly lines, and the positioning of work pieces or tools for operations such as punching, drilling, printing, clamping and so forth. The devices can also be used to position individual parts for automatic assembly, etc. In each of these jobs, repetitive, precise and accurate movement in the face of undesired external loads is essential.
Pneumatic and hydraulic operated fluid devices accomplish movement of a tool or work piece by a power mechanism acting on a tooling plate. One conventional power mechanism includes a double action piston located within a cylinder and integrally connected to a piston rod. Pneumatic or hydraulic pressure is applied to either side of the piston so that a pressure differential is created across the piston. The differential pressure in the cylinder controls the location of the piston. It causes the piston to displace within the cylinder until the force on both sides of the piston is equal. The displacement, or stroke, of the piston rod is generally limited to the distance the piston can displace within the cylinder. This type of a system can be disadvantageous if the fluid medium is compressed air and the piston is floating in the cylinder and finally positioned by equal fluid forces being established on opposite sides of the piston. In heavy machine tool work, the forces created between the tools and the work can add to the force on one side of the piston within the cylinder, upsetting the equilibrium and throwing the tool out of alignment.
One manner of overcoming this disadvantage has been to utilize a plurality of fluid-actuated cylinders, such as hydraulic cylinders that do not rely on the establishing of equilibrium pressure. These cylinders have piston strokes of varying lengths and are stacked in an end-to-end relationship to provide a more rigid connection between the controlled tool and the positioning device. Such a device is disclosed in U.S. Pat. No. 3,633,465 to Puster. The actuated pistons disclosed in Puster slide the cylinders a distance that is equal to the sum of the stroke lengths of each actuated cylinder. Sizing the cylinders so that each has a different stroke length allows the device to achieve a large number of positions. Conventional multi-stroke, actuated cylinders are not laterally stable and occupy an excessive amount of space during use. In addition, many of these conventional actuators utilize position feedback mechanisms for insuring the accuracy of the positioning of the tooling plate. Typically, these feedback mechanisms include sensitive electrical feedback loops that can cause radio frequency interference with the power and fluid control mechanisms. Also, the use of electrical feedback or position control mechanisms can require shaft encoders that impose a risk of sparks or shorts, thereby creating explosive or otherwise hazardous conditions.
It is an object of the present invention to overcome the disadvantages of the prior art. It is also an object of the present invention to provide a multi-stroke cylinder capable of accurately achieving a large variety of positions without the use of a position feedback mechanism.
The present invention relates to a multi-stroke air cylinder that provides a precisely directed and controlled stroke in the face of lateral, torsional and tilting loads on a tooling plate. The present invention can use binary techniques or combinations of stroke increments to provide a precise positioner utilizing pneumatic or hydraulic power that provides accurate positioning of a tool without requiring or using position feedback mechanisms. Also, the air cylinder is laterally stable so it can be used in areas such as woodworking, apparel manufacturing, building materials, housing construction and other similar arts.
The present invention utilizes a plurality of mechanically linked pneumatic or hydraulic pistons having different stroke lengths that can be added together in any combination, allowing the user to select any stroke length up to a predetermined, total combined stroke length, in increments equal to the stroke length of the shortest stroke piston. For example, if the invention included four pistons having stroke lengths of one inch, two inches, four inches and eight inches, the user can select any stroke length in increments of one inch up to a total combined stroke length of fifteen inches. A three inch stroke would be obtained by extending the one inch stroke piston and the two inch stroke piston. A seven inch stroke would be obtained by extending the one inch stroke piston, the two inch stroke piston and the four inch stroke piston. The activation and extension of all of the pistons would achieve a fifteen inch stroke. The present invention also includes a plurality of pistons that can move the tooling plate by a fraction of an inch. This fractional movement can be added to the movement of the pistons having full inch increments so that positions in increments of the smallest fraction of an inch can be achieved up to the aggregate stroke length of all of the pistons.
The multi-stroke cylinder according to the present invention includes a head assembly having a fluid inlet for introducing fluid to the cylinder at a first pressure. The cylinder also includes a first positioning system having a plurality of pistons capable of moving the piston rod away from the first positioning system. A second positioning system is located between the head assembly and the first positioning system. The second positioning system comprises a plurality of movable pistons for moving the piston rod a preselected distance and a plurality of fluid supply members which are each secured to a respective one of the pistons of the second positioning system for introducing a fluid between adjacent pistons. The fluid supply members are concentrically arranged and are at least partially coextensive with one another. The disadvantage previously discussed concerning differential pressure pistons does not occur with the present invention because an equilibrium is not established. Instead, low pressure used to maintain the rest position of the pistons is expelled from the cylinder of the second positioning system as the piston is moved by the higher pressure introduced through the fluid supply members.
The first or xe2x80x9cfinexe2x80x9d positioning system utilizes a plurality of positioning stages having increments of movement in {fraction (1/16)} of an inch intervals up to a total of {fraction (15/16)} of an inch. The smallest of the different sized stages is {fraction (1/16)} of an inch. The second or xe2x80x9ccoarsexe2x80x9d positioning system has increments of movement set in one inch intervals up to a total of fifteen inches. In this system, the pistons would be set to extend at different lengths with the smallest stage length being one inch. By activating the coarse and fine positioning systems, the tooling plate of the present invention can be positively positioned in as many as 256 individual positions. If an additional stage capable of {fraction (1/32)} of an inch were added, the number of discrete positions that could be achieved would be doubled to 512, thereby increasing the accuracy of the multi-stroke cylinder. Similarly, adding another stage capable of {fraction (1/64)} of an inch movement could again double the accuracy while quadrupling the original number of discrete positions obtainable to 1024.
The present invention accurately positions the head of a piston rod or other similar devices such as a tooling plate in one, two or three planes by activating one or a plurality of pistons within a cylinder. Valves control the flow of the fluid medium within the cylinder and between the pistons. The head of the tooling piston or plate can securely and accurately carry any number or types of tools for performing an application on a work piece. For instance, by attaching a drill, the user could accurately drill a hole anywhere in an X-Y plane to a depth of Z and repeat the same controlled drilling depth at a second location. Alternatively, the hole could be drilled to a different depth at the second location. By attaching a parts gripper, the operator could retrieve a part from a known inventory position and place it accurately in an assembly a predetermined distance away. The present invention allows these applications to occur without the forces generated at the work piece affecting the position of the head of the piston rod.
Unlike conventional multi-stroke actuators and their related methods for carrying out the above discussed tasks, the embodiments according to the present invention do not require a feedback mechanism to insure the positioning accuracy of the tooling piston or plate. Selecting the proper combination of valves insures that the piston rod moves positively to the selected position. An additional advantage arises from the exclusive use of fluid power to carry out the positioning, thereby eliminating the necessity of employing electrical counters or shaft encoders which impose the risk of sparks or shorts in explosive or otherwise hazardous conditions. Furthermore, the present invention is completely free of radio-frequency interference since no sensitive electrical feedback loops are required. The multi-stroke cylinders according to the present invention are also compact in size and laterally stable so that they are able to be used in a variety of locations for performing many different operations.